1. Field of the Invention
This invention relates to a plastic objective lens system wherein fluctuation in optical characteristics due to a temperature change is reduced by use of a plurality of plastic materials exhibiting different coefficients of refractive index change with temperature.
2. Description of the Prior Art
It is known for positive power lenses constituted of plastic materials that, as the temperature rises, the focal lengths generally increase and the image formation surfaces deviate since the refractive indices of the plastic materials become small and the radii of curvature of the surfaces of the lenses become large due to thermal expansion of the plastic materials.
In order to correct for fluctuation in optical characteristics, it has been proposed, for example in Japanese Unexamined Patent Publication Nos. 57(1982)-20507 and 57(1982)-20508, to change the surface separations of lenses according to temperature.
However, the conventional system for changing the surface separations of lenses for the purpose of the correction for movement of the image formation surface due to a change in temperature requires a complicated lens barrel configuration, a high-accuracy assembly technique, and a large space of the lens system. As a result, the advantages of the use of plastic materials with regard to the manufacture cost and manufacture processes are lost.
Further, though the conventional system can correct the positions of the image formation surfaces, it has drawbacks that the optical performance is readily deteriorated since aberration such as distortion of image surfaces is aggravated, and that the optical axes of lenses are deviated from one another when the surface separations of the lenses are corrected. Thus the conventional system involves many problems to be solved for putting it into practice.
Particularly in a system wherein the image formation magnification is finite and the positions of the object, the image formation surface and the lenses are fixed, for example, in an objective lens system for copying or projection, large fluctuation in focal lengths results in large deviation of the position of image formation. Further, even though the position of image formation is corrected by moving the lens system, the magnification is fluctuated markedly. Therefore, in order to put the system into practice, it is necessary to effect correction for optical characteristics at a high accuracy with respect to temperature.